FEW ever gave a second look to the block of flats in St Peter's Street, Huddersfield.

There was nothing unusual to point it out from the surrounding properties.

So it was a major surprise when police raided the town centre building.

Inside, they found a huge cannabis factory, with 750 plants growing in many of the rooms.

A canopy of lights and a watering system had been set up to tend the plants.

Police estimated that the drugs haul had a street value of £250,000.

"We had no idea this was going on," said a shocked neighbour, who asked not to be identified.

Five men found inside the house were arrested and three of them were later remanded in custody, charged with conspiracy to produce cannabis.

Cannabis cultivation is booming in Britain, with production sites being raided at the rate of at least three a day over the last six months. More than 1,500 cannabis farms have been closed down in London alone in the last two years, three times the amount shut down between 2003-2005.

The research, published by the charity DrugScope, reveals the staggering growth.

Just 10 years ago, only 11% of cannabis sold in the UK was grown here.

That figure now stands at 60%.

Police say that 80% of farms have more than 50 plants, with an average of 400 plants recovered per raid. Most set-ups are using simple growing systems - overhead lights, fans and plant pots - with fewer than one in 10 farms using more expensive and technical systems.

A police swoop earlier this month on a rented house in Waverley Terrace, Marsh, showed the increasing extent of the problem in Huddersfield.

It was the fifth cannabis farm to be uncovered in the town this year.

Three Vietnamese men were arrested at the two-bedroom terraced house.

There's little doubt that more cannabis farms will be discovered - for the crop has become a very valuable product.

Det Sgt Anil Shukla, from Kirklees Police Drugs Team, is part of a team uncovering these new enterprises.

Police are encouraging estate agents and landlords to beware of people looking to pay cash for tenancy agreements.

Det Sgt Shukla says: "We are urging people to look out for any suspicious activity in properties near them.

"They should be wary of things like people coming and going at odd times and unusual equipment being brought in, together with bags of compost and fertiliser.

"We want people to let us know because it could be a sign that large-scale cannabis growth is going on behind the doors.

"Cannabis production is a serious offence and offenders should be brought to justice."

Analysis of nationwide police raids reveals around two-thirds to three-quarters of cannabis farms are run by Vietnamese criminal gangs.

Initially thought to be restricted to the Greater London area, in the last year Vietnamese-run cannabis farms have been found in many other parts of the country.

Many of the growers are illegal immigrants coerced into living in cramped conditions as payback for the gangs which brought them into the country.

Police have found growers living in cupboards, tiny utility rooms and lofts to maximise space for plants.

The farms have risen in number because of a slump in cannabis coming in from abroad, due to Government clampdowns.

Supplies of Moroccan cannabis - previously a key source - have fallen by almost a half in three years.

Contrary to many reports, gangs usually opt for high-yield plants, with an early flowering time, rather than growing super strength skunk.

Harry Shapiro, of DrugScope says: "Growing cannabis commercially near the point of sale can dramatically increase profits, but this increases the risk of detection."

But police face a constant battle to outwit growers who are becoming increasingly smart in establishing new farms, reckons Mr Shapiro.

Other high-profile raids this year in Huddersfield include:

* April - Police unearth 400 plants worth more than £100,000 at a terraced house in Wakefield Road, Aspley.

* April - Vietnamese man arrested at house in Church Street, Paddock, where there were 130 skunk plants worth £40,000.

* March - Hundreds of plants worth £250,000 seized at house in Denby Dale.

* February - Cannabis operation found at two houses in Manor Road, Newsome, after one of the properties caught fire.